Wheat responses to arbuscular mycorrhizal fungi in a highly calcareous soil differ from those of clover, and change with plant development and P supply

We evaluated the roles of arbuscular mycorrhizal (AM) fungi in growth and phosphorus (P) nutrition of wheat (Triticum aestivum L.) in a highly calcareous soil and compared the responses of wheat with those of clover (Trifolium subterraneum L). In the first experiment wheat (cv. Brookton) was harvested at 6 wk. Colonisation by four AM fungi was low (< 20%). Clover was harvested at 8 wk. Colonisation varied with different fungi, with the highest value (52%) obtained with Glomus intraradices. Although suffering from P deficiency, non-mycorrhizal (NM) wheat grew relatively well with no added P (P0) and application of P at 100 mg kg(-1) (P100) increased the dry weight (DW). Shoot P concentrations increased with P application and there were positive effects of all AM fungi at P100. In contrast, NM clover grew very poorly at P0 and did not respond to P application. Clover responded positively to all AM fungi at both P levels, associated with increases in P uptake. In the second experiment colonisation by a single AM fungus (G. intraradices) of two wheat cultivars (Brookton and Krichauff) was well established at 6 wk (similar to 50% in P0 plants) and continued to increase up to maturity (similar to 70%), but decreased greatly at both harvests as P supply was increased (up to 150 mg P kg(-1): P150). Addition of P significantly increased plant growth, grain yield and P uptake irrespective of cultivar and harvest time, and the optimum soil P for grain yield was P100. In both cultivars, a growth depression in AM plants occurred at 6 wk at all P levels, but disappeared at 19 wk with added P. At P0, AM plants also produced lower grain yield (weight) per plant, but with higher P, AM plants produced higher grain yields than NM plants. There was a significant positive effect of AM on grain P concentration at P0, but not at other P levels. Brookton was somewhat more P efficient than Krichauff, and the latter responded more to AM fungi. This study showed that responses of wheat to AM inoculation and P supply were quite different from those of clover, and changed during development. Results are discussed in relation to the underlying soil properties.